Abstract forward selection to its own R package

[ttimbers]

Abstract the best subset selection code in the feature selection section to its own R package. When doing this, it would be good to try to make it compatible/stylistically work well with tidymodels. Also, we might consider implementing forwards and backwards selection as well.

[trevorcampbell]

Update on this -- for various reasons ( see #146 ) we have just switched to forward selection. Still need to make it its own R package though, because the code is way too complicated for the level of our readers.

[trevorcampbell]

OK I'm going to merge PR #146 prior to @ttimbers finishing abstracting that code into a package. That can be its own subsequent PR.

[trevorcampbell]

Perhaps we could mostly copy the code pattern here:

https://github.com/tidymodels/tune/blob/034b75a1b032bd29ceb70df8d4e3ce10e9fcc17b/R/tune_grid.R#L326

which uses tune_grid_loop from here:

https://github.com/tidymodels/tune/blob/034b75a1b032bd29ceb70df8d4e3ce10e9fcc17b/R/grid_code_paths.R#L1

??